In the various kraft pulp processes cellulosic material or chips are generally treated at elevated temperatures with alkaline cooking liquor containing sodium hydroxide and sodium hydrogen sulfide. In these processes, fresh cooking liquor is generally referred to as white liquor, and spent liquor is generally referred to as black liquor.
On a chemical basis, the kraft pulp process used industrially is the same today as was the case one hundred years ago. While it is true that many different chemical means have been proposed for the purpose of improving factors such as the yield and selectivity of the processes, none of these proposals has led to acceptable practical solutions to these problems because each of them has entailed complicated equipment, additional process steps or the use of expensive chemicals.
In addition, different chemical methods for the pretreating of chips have also been proposed. Many of these proposed chemical pretreatment methods have been based upon the use of hydrogen sulfide or bisulfide. For example, Finnish Patent No. 29611 describes a pretreatment process utilizing hydrogen sulfide under elevated pressure. Also, Swedish Patent No. 309530 relates to a pretreatment process utilizing liquid hydrogen sulfide at a pH of between 4 and 10. Polysulfide treatment has also been proposed as a second pretreatment step.
The kraft process, however, has been developed by means of different technical processing means. In particular, the need to save energy has led to new solutions, the most important of which have been continuous cooking processes (see, e.g., Finnish Patent No. 54155). The equipment used in such continuous cooking processes can include the use of several co- and countercurrent circulations, as well as separate impregnation vessels.
Batch processes have also been developed for the purpose of saving energy. In many of the processes which have thus been developed, hot black liquor is displaced from the digester prior to discharge. This displaced liquor is then used for preheating the chips, or as cooking liquor in subsequent batches (see, e.g., U.S. Pat. No. 4,578,149 and Finnish Laid Open Publication No. 71176).
It has also been proposed to improve the quality of the pulp being produced by avoiding digester discharge which utilizes hard hot blow techniques. This can be accomplished by using the cold blow method (see, e.g., Finnish Patent Application No. 791205), or by means of pump discharge (see, e.g., U.S. Pat. No. 4,814,042).